Catherine, this question always sounds simpler to answer than it is, because the subject of resolution can be hard to grasp for many people, especially when the discussion starts hitting on resampling. Here's a link to a very good site that will give you some basic information on image resolution. The link should open to a page where a lot of topics are listed; look down the list until you find the part about image resolution.
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http://www.arraich.com/elements/pse_hhow1.htm>
I'm not really intending to hang you out to dry with just a link, though. Read through the information on Jay's site and then post back with specifics about the picture you're working with. If we have something concrete to deal with, the discussion will be a little less confusing.
Put the original up on your monitor and then go to Image>Resize>Image Size and post the dimensions of the image and the current resolution. Also give us some idea of how much of the original picture you want to crop out and enlarge, like one half of it, a third, a quarter, etc. That will have a significant impact on the resolution, and it's the resolution that's much more critical to the quality of the print than the printer settings.